There is currently a problem in the nuclear field, and probably in other fields, where a leak at a tube in a nest of tubes where it is welded to a tube plate in a heat exchanger has to be made good without gaining access to the outside of the tube. The known practice for dealing with this problem is to insert a length of repair tube into the leaking tube through the tube plate with one (the outer) end of repair tube explosively welded to the tube plate and the other (the inner) end brazed to the leaking tube.
We have now discovered that the quality of the braze can be effected by the prescnce of stress in the tube being repaired, such stress arising during the heating necessary to effect the braze.
Said application Ser. No. 596,720 of even date in the names of Roach et al provides a method of repairing a leaking heat exchanger tube at its weld with a tube plate comprising the insertion of a length of repair tube into the leaking tube through the tube plate with the outer end of the repair tube explosively welded to the tube plate and the inner end brazed to the leaking tube characterised in that, prior to effecting the repair, the leaking tube is subjected to a strain-on-heating test.
Said copending application also provides apparatus for carrying out the above method.
Investigating the strain-on-heating matter further we have found that some degree of strain always exists. A basic strain arises from the fact that one leg of a U-shaped heat exchanger tube is being heated whilst the other leg is not. The magnitude of this basic strain tends to be related to the spacing apart of the legs of the U-shape: a wide spacing gives low strain whilst a close spacing gives a higher strain. Strains can also arise on heating due to binding between a tube and tube support plate.